Grinding apparatus and method

ABSTRACT

An apparatus for grinding a workpiece on a support surface includes a rotatable and vertically movable grinding wheel having an abrading surface and at least one cylinder coupled to the grinding wheel. The cylinder includes a piston, a stepper-motor coupled to the piston, and a converter coupled to the piston and to the stepper-motor for converting rotary movement of the stepper-motor to linear movement of the piston to vertically move the grinding wheel. A measuring device provides a representation of the distance between the support surface and the abrading surface. A controller is coupled to the measuring device and to the stepper-motor for receiving the representation and applying an activation signal to the stepper-motor to vertically move the grinding wheel to reach a predetermined distance between the support surface and the abrading surface.

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/450,242, filed Feb. 25, 2003.

FIELD OF THE INVENTION

[0002] The present invention relates generally to a method and apparatusfor grinding a workpiece to achieve a desired workpiece dimension, andmore particularly to a thru-feed grinding apparatus utilizing animproved closed-loop, feedback control system resulting in enhanced sizecontrol.

BACKGROUND OF THE INVENTION

[0003] There are many varieties of grinding machines; for example,horizontal-spindle, reciprocating-table surface grinders; double-discgrinders; and abrasive belt grinders. A thru-feed grinder is a veryefficient apparatus for the high production surface grinding ofworkpieces because it requires little fixturing and set-up time andprovides for the continuous loading and unloading of workpieces. Thatis, because thru-feed grinders employ a conveyor feed assembly,workpieces are fed to the grinder on a continuous basis thus permittingvirtually continuous grinding.

[0004] Certain traditional thru-feed grinders are equipped withhydraulic cylinders that support the grinding wheel. Such grinders,however exhibit certain shortcomings related to size control stability.That is, over time the hydraulic cylinders may drift resulting inchanges in the distance between the chuck (i.e. the surface supportingthe workpieces) and the working surface of the grinding wheel. The abovedescribed drift occurs for three primary reasons. First, it is extremelydifficult to bleed all air from the hydraulic system. Second, thehydraulic cylinders are typically not completely leak-proof, and third,hoses coupled to the hydraulic cylinders are generally flexible and willexpand with increasing pressure. Drift can result in dimensionalvariations in the processed workpieces, and if the drift exceeds acertain value, the system may lift the grinding wheel from the part in arelatively uncontrolled manner requiring a very precisely controlledsubsequent downward movement of the grinding wheel to compensate forovershoot. Events such as this cannot be tolerated in the production ofparts with high dimensional tolerances. Furthermore, the problem ofachieving high-tolerance precision grinding is exacerbated by thewearing down of the grinding wheel with time as abrading material on thegrinding wheel is consumed.

[0005] Thus, it would be desirable to provide a precision thru-feedgrinding apparatus employing a closed-loop feedback control system thatsubstantially avoids the problems associated with the above describeddrift. Furthermore, other desirable features and characteristics of thepresent invention will become apparent from the subsequent detaileddescription of the invention and the appended claims, taken inconjunction with the accompanying drawings and this background of theinvention.

BRIEF SUMMARY OF THE INVENTION

[0006] According to an aspect of the invention there is provided anapparatus for grinding a workpiece on a support surface. The apparatuscomprises a rotatable and vertically movable grinding wheel having anabrading surface and at least one cylinder coupled to the grindingwheel. The cylinder includes a piston, a stepper-motor coupled to thepiston, and a converter coupled to the piston and to the stepper-motorfor converting a rotary movement of the stepper-motor to linear movementof the piston to vertically move the grinding wheel. A measuring deviceprovides a representation of the distance between the support surfaceand the abrading surface. A controller is coupled to the measuringdevice and to the stepper-motor for receiving the representation andapplying an activation signal to the stepper-motor to vertically movethe grinding wheel to achieve a predetermined distance between thesupport surface and the abrading surface.

[0007] According to a further aspect of the invention there is providedan apparatus for grinding a workpiece on a support surface. Theapparatus comprises a rotatable and vertically movable grinding wheelhaving an abrading surface, and a feedback control network formaintaining a predetermined distance between the abrading surface andsupport surface. The feedback control system comprises astepper-motor-controlled hydraulic cylinder coupled to the grindingwheel for vertically moving the grinding wheel, a measuring device forindicating when the distance between the support surface and theabrading surface is different than a predetermined distance, and acontroller is coupled to the measuring device and to thestepper-motor-controlled hydraulic cylinder for activating the cylinderto vertically move the grinding wheel to achieve the predetermineddistance.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The present invention will hereinafter be described inconjunction with the following drawing figures, wherein like numeralsdenote like elements, and

[0009]FIGS. 1 and 2 are isometric views of a thru-feed grindingapparatus in accordance with one embodiment of the present invention;

[0010]FIG. 3 is a plan view of the conveyor assembly and grinding wheelutilized in the apparatus shown in FIG. 1;

[0011]FIG. 4 is an isometric view of an pneumatic gauge assemblyutilized in the apparatus shown in FIG. 1;

[0012]FIG. 5 is an isometric view of the slide-posts, dampeners, andplates utilized in the apparatus shown in FIG. 1;

[0013]FIG. 6 is a schematic diagram of a grinding apparatus inaccordance with the present invention; and

[0014]FIG. 7 is a schematic diagram of a pneumatic air gauge for use inconjunction with the apparatus shown in FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION

[0015] The following detailed description of the invention is merelyexemplary in nature and is not intended to limit the invention or theapplication and uses of the invention. Furthermore, there is nointention to be bound by any theory presented in the precedingbackground of the invention or the following detailed description of theinvention.

[0016]FIG. 1 and FIG. 2 are isometric views of a thru-feed grindingapparatus 10 in accordance with an embodiment of the present invention.The apparatus comprises a lower housing 12, an upper housing 14, acontinuous conveyor assembly 16 powered by a motor (not shown) withinlower housing 12, guide rails 18 and 20, and a grinding wheel 22 mountedfor both vertical and rotational movement to an upper carriage assembly(not shown) within upper housing 14. Grinding wheel 22 may be astandard, inserted-nut, disc wheel mounted on a vertical spindle (65 inFIG. 6) over conveyor assembly 16. Conveyor assembly 16 includes amotor-driven, continuous conveyor belt 24 (preferably an abrasive belt)which passes over a magnetic table 26 in a direction indicated by arrow28. Magnetic table 26 may comprise a variable-power, electromagnetictable that serves as a locating table and magnetic chuck.

[0017] Conveyor assembly 16 is preferably a variable speed system andimparts translational movement to conveyor belt 24 that is of sufficientwidth (e.g. six inches) to carry workpieces 30, placed on the belt by anoperator 36, beneath grinding wheel 22 and through the grindingoperation until the workpiece is off-loaded at the downstream end of thebelt. As can be seen in FIG. 2, grinding wheel 22 is configured forrotation as indicated by arrow 32 and for vertical translation asindicated by arrow 34 in a manner to be described herein below. In thismanner, a workpiece 30 passes under grinding wheel 22, and a surface ofthe workpiece is ground to a desired dimension. Guide rails 18 and 20are provided to position the workpiece 30 on conveyor belt 24 and absorbthe side thrust of grinding wheel 22. The guide rails are adjustable andassist in orienting and directing the workpieces by allowing thetorsional thrust of the grinding wheel to automatically position theworkpieces against the rails while at the same time prevent theworkpieces from being swept off the conveyor. The guide rails alsoprevent tipping of the workpieces as they pass under the grinding wheelthus permitting the operator to simply and continuously place workpieceson the conveyor.

[0018] Referring to FIG. 3, workpieces 30 loaded onto conveyor belt 16are carried under the working surface of grinding wheel 22 and ground asthey are held in position by the magnetic table 26 and adjustable guiderails 18 and 20. Finished parts 48 are discharged at the opposite end 40of conveyor 24 into a suitable container or, if desired, to subsequenthandling equipment. A longitudinal inclination of the magnetic table(e.g. 0.0015 inches per inch) permits the workpieces 30 to be groundonly when entering the grinding wheel area and pass through the center42 and trailing section 44 of the grinding wheel 22 untouched. Thiscauses a slight taper 46 on the face of the grinding wheel proportionalto the inclination of the magnetic table 26. In this manner, grindingwheel 22 is continuously dressed by the workpieces. Workpiece dimensionis determined by the distance 50 between the abrading surface of wheel22 and the surface 52 upon which the workpiece is supported.

[0019] In short, the grinding process is accomplished by four elements;(1) magnetic table 26 that serves as a locating surface and magneticchuck, (2) conveyor belt 24 that moves workpieces 30 through thegrinding operation, (3) rotating grinding wheel 22 that imparts avertical force on workpieces 30 to keep them securely position on thelocating surface or conveyor belt; and (4) adjustable guide rails 18 and20 against which the workpieces are firmly held and which absorb thegrinding torsional force of the grinding wheel.

[0020] Housings 12 and 14 are made of any material (e.g. steel) ofsufficient strength to withstand the strain of heavy stock materialwhile at the same time provide for smooth operation. Referring to FIG.5, a plurality (e.g. four) vertical slide posts 54 support grindingwheel 22, motorized spindle 65 (FIG. 6) and a hydraulic feed mechanism(74 and 76 in FIG. 6). All sub-assemblies are mounted on heavy supportplates (e.g. 56) which form an integral structural unit with posts 54.Dampeners 58 under support plates 56 reduce machine-to-workheadvibrations

[0021]FIG. 6 is a schematic diagram of a grinder apparatus in accordancewith the present invention wherein like elements are denoted by likereference numerals. An operator places workpieces on conveyor 24 whichmoves in the directions indicated by arrows 60 to bring the workpiecesbeneath grinding wheel 22 as described above. Conveyor 24 is driven by aconveyor motor 62, and grinding wheel 22 is rotated by motor 64 asindicated by arrow 66. Grinding wheel 22 is coupled to a support plate68 which is configured to slide vertically on posts 54 as is indicatedby arrow 70. Posts 54 are coupled at their upper ends to a top plate 72.

[0022] Mounted above support plate 68 are a plurality (e.g. two) ofstepper-motor-controlled high-precision hydraulic cylinders 74 and 76.Each precision hydraulic cylinder includes a cylinder shaft 78 housing apiston 80 coupled to a piston rod 82 that extends through openings 84 intop plate 72 so as to move wheel support plate 68 vertically on posts54. As can be seen, each cylinder includes a stepper-motor 86, a spooland valve assembly 88 and a rotation-to-translation converter 90. Spooland valve assemblies 88 are coupled to a source of pressure 92 and, viaconduits 94 and 96, to the inner regions of shaft 78 on opposite sidesof piston 80.

[0023] Stepper-motors 86 are electrically coupled to a programmablelogic controller 98 which is in turn configured to receive a measurementsignal from pneumatic gauge 100, to be more fully described below.Simply stated, as grinding wheel 22 grinds workpieces on conveyor 24, asmall amount of abrading material is lost on the abrading surface ofgrinding wheel 22. Pneumatic gauge 100 monitors the distance between theabrading surface of grinding wheel 22 and the surface upon which theworkpieces are resting (i.e. conveyor 24), and when this distanceexceeds a predetermined value due to the loss of abrading material ongrinding wheel 22, controller 98 activates stepper-motors 86 which inturn causes pistons 80 to move downward and, via piston rods 82, to movewheel support plate 68 downward so as to achieve a desired spatialrelationship between the abrading surface of grinding wheel 22 and theupper portion of conveyor 24 upon which the workpieces are loaded. Thatis, the activation signal provided by controller 98 to stepper-motors 86will cause the stepper-motor shafts to rotate in very preciseincrements. The stepper-motor shafts operate on an internal spool andvalve assemblies 88 imparting rotary and linear movement to the spooland valve assemblies 88 and the appropriate closure and opening ofvalves to provide fluid pressure to cylinder shafts 78. Rotation of thespool is translated to linear movement in rotary-to-translationconverters 90 to move pistons 80 vertically in an appropriate direction.This may be accomplished by a ball nut attached to each piston 80 thatrotates a ball screw directly coupled to the valve spool. In thismanner, the speed of pistons 80 is positively synchronized to therotational speed of the stepping motor. The piston continues rotatingthe spool until a shut-off position is reached (i.e. when thepredetermined spacing between the abrading surface of grinding wheel 22and the upper portion of conveyor 24 has been reached). Thus, thedigitally operated rotating stepping motors 86, cylinders 74 and 76,grinding wheel 22, pneumatic gauge 100 and controller 98 form aclosed-loop feedback control system. Each step of the stepping motors 86is very precise and therefore very accurate positioning of grindingwheel 22 over conveyor 24 results. Stepping motors 86 may beincrementally rotated in either direction depending upon the manner inwhich the signal from controller 98 is applied. In the absence of anyactivation signal from controller 98, the cylinder is inherently brakedand maintained stationary. A more detailed discussion of high-precisiondigitally, controlled hydraulic cylinders may be found in U.S. Pat. No.3,457,836 issued Jul. 29, 1969 and entitled “DIGITALLY OPERATEDELECTROHYDRAULIC POWER SYSTEM” assigned to The Superior ElectricCompany, Bristol, Conn. Such devices are also commercially availablefrom Fluid Power Technology, located in Charlotte, N.C.

[0024]FIG. 7 is a schematic diagram of a pneumatic gauge suitable foruse at air gauge 100 in FIG. 6. The pneumatic (air) gauge provides forcontinuous and automatic compensation for wear on grinding wheel 22. Itmaintains a substantially constant dimension between the surface onwhich workpieces 30 rest (i.e. conveyor belt 24) and the abradingsurface of the grinding wheel. This distance corresponds to the grounddimension of the finished part. Once established, workpiece thickness ismaintained until all usable abrasive in the grinding wheel is consumed.At this point, a controller coupled to the air gauge automatically shutsthe system down and generates an alert or warning (e.g. illuminates alight on a control panel). It operates on the principals of air flow atconstant velocity and a pneumatic wheatstone bridge. The device may beconsidered an air to electric converter and is fed by a single airsupply line 102 which is divided into two parallel paths 104 and 106.Air lines 104 and 106 are separated by an extremely flexible, air-tightdiaphragm 108. Lower air line 106 is referred to as a measurement lineand comprises a calibrated opening or measurement nozzle 110 and ameasurement opening 112 which is the resultant opening produced when thecalibrated air jets (or variable restricting device of a gauge) iscombined with the workpiece or master. The upper airline 104 is anadjustment or balance line and comprises a calibrated opening or balancenozzle 106 and an opening 114 (e.g. an annular orifice) which permitsair to escape to the atmosphere at a rate dependant upon the position ofa tapered needle 116 with respect to outlet 114. A balance orequilibrium condition exists when there is substantially equal pressurein both the balance line 104 and the measurement line 106.

[0025] Any increase in pressure in the measurement line 106 will propelthe diaphragm 108 upward thus moving needle 116 upward until the annularoutlet 114 around needle 116 is such that the pressure in both upperline 104 and lower line 106 is substantially equal. An opposite effectwould occur if pressure were to drop in measurement line 106. A distancemeasurement between the lower surface of the grinding wheel and theupper surface of the conveyor is related to the displacement of needle116 acting on plunger 118 and indicator 120 in relation to the originalposition which was determined when the instrument was calibrated againsta known dimension, part, or master. This measurement may be read bycontroller 98 (FIG. 6) via an electric probe. The displacementmeasurement provided by air gauge 100 to controller 98 is then convertedin controller 98 to energize stepper-motors 86 to increment ordecrement. As a result, cylinder 74 and 76 vertically move wheel plate68 until the appropriate dimension has been reached at which pointcontroller 98 terminates activation of stepper-motors 86.

[0026] Thus, there has been provided a precision thru-feed grindingapparatus that avoids the problems associated with drift and compensatesfor loss of abrading material on the grinding wheel through the use ofhigh precision digital cylinders and a closed look feedback system.

[0027] While at least one exemplary embodiment has been presented in theforegoing detailed description of the invention, it should beappreciated that a vast number of variations exist. It should also beappreciated that the exemplary embodiment or exemplary embodiments areonly examples, and are not intended to limit the scope, applicability,or configuration of the invention in any way. Rather, the foregoingdetailed description will provide those skilled in the art with aconvenient roadmap for implementing an exemplary embodiment of theinvention, it being understood that various changes may be made in thefunction and arrangement of elements described in an exemplaryembodiment without departing from the scope of the invention as setforth in the appended claims.

What is claimed is:
 1. An apparatus for grinding a workpiece on asupport surface, the apparatus comprising: a rotatable and verticallymovable grinding wheel having an abrading surface; at least one cylindercoupled to said grinding wheel, said cylinder comprising: a piston; astepper-motor coupled to said piston; and a converter coupled to saidpiston and to said stepper-motor for converting rotary movement of saidstepper-motor to linear movement of said piston to vertically move saidgrinding wheel; a measuring device for providing a representation of thedistance between said support surface and said abrading surface; and acontroller coupled to said measuring device and to said stepper-motorfor receiving said representation and applying an activation signal tosaid stepper-motor to vertically move said grinding wheel to achieve apredetermined distance between said support surface and said abradingsurface.
 2. An apparatus according to claim 1 wherein said supportsurface is continuously movable beneath said grinding wheel.
 3. Anapparatus according to claim 2 wherein said support surface is aconveyor belt.
 4. An apparatus according to claim 3 wherein saidconveyor belt is inclined downward in the direction of movement of saidsupport surface beneath said grinding wheel.
 5. An apparatus accordingto claim 3 wherein said measuring device is a pneumatic gauge.
 6. Anapparatus according to claim 5 wherein said pneumatic gauge ispositioned between said abrading surface and said support surface.
 7. Anapparatus according to claim 6 wherein said pneumatic gauge ispositioned proximate an outer periphery of said grinding wheel.
 8. Anapparatus according to claim 7 wherein said controller is a programmablelogic controller.
 9. An apparatus according to claim 8 wherein saidcontroller terminates activation of said stepper-motor when the distancebetween said support surface and said abrading surface reaches apredetermined distance.
 10. An apparatus for grinding a workpiece on asupport surface, the apparatus comprising: a rotatable and verticallymovable grinding wheel having an abrading surface; and a feedbackcontrol network for maintaining a predetermined distance between saidabrading surface and said support surface, said feedback control systemcomprising: a stepper-motor controlled hydraulic cylinder coupled tosaid grinding wheel for vertically moving said grinding wheel; ameasuring device for indicating when the distance between said supportsurface and said abrading surface is different than a predetermineddistance; and a controller coupled to said measuring device and to saidstepper-motor controlled hydraulic cylinder for activating said cylinderto vertically move said grinding wheel to reach said predetermineddistance.
 11. An apparatus according to claim 10 wherein said controlleractivates said cylinder to lower said grinding wheel as it becomesthinner due to wear.
 12. An apparatus according to claim 10 wherein saidmeasuring device is a pneumatic gauge.
 13. An apparatus according toclaim 12 wherein said pneumatic gauge is positioned between saidabrading surface and said support surface.
 14. An apparatus according toclaim 13 wherein said pneumatic gauge is positioned proximate an outerperiphery of said grinding wheel.
 15. An apparatus according to claim 14wherein said controller is a programmable logic controller.
 16. Anapparatus according to claim 13 wherein said support surface iscontinuously movable beneath said grinding wheel.
 17. An apparatusaccording to claim 16 wherein said support surface is a conveyor belt.18. An apparatus according to claim 13 wherein said stepper-motorcontrolled hydraulic cylinder comprises: a piston; a stepper-motorcoupled to said piston; and a converter coupled to said piston and tosaid stepper-motor for converting rotary movement of said stepper-motorto linear movement of said piston to vertically move said grindingwheel.